DOT set to make final round of repairs on cracked 520 pontoons

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»Play VideoThe coffer cell is a large steel structure that measures 35 feet tall by 45 feet wide by 144 feet long. It was built in Tacoma and towed to Lake Washington on a barge. Once in place, the coffer cell will be used by SR 520 bridge construction crews to repair two concrete pontoons on the lake. (Photo: WSDOT)

ON LAKE WASHINGTON, Wash. -- Washington's Department of Transportation is set to start the final round of repairs on cracked and leaking pontoons for the 520 bridge. The Problem Solvers first exposed those serious issues in an investigation last year. This massive undertaking will use a 650-ton steel box called a coffer cell to complete the repairs.

The coffer cell is the final piece of the puzzle for fixing the last two pontoons on the lake that still need significant repairs. Ultimately, 77 of these massive concrete pontoons will reach across Lake Washington and form the floating base of the new bridge.

On a boat tour of the construction site Friday, WSDOT personnel pointed out pontoons W and T, which were repaired in dry dock last summer and moved back onto the lake. When asked how if those repairs fixed the leaks, WSDOT Bridge Construction Manager Dave Becher told us, "we're not aware of anything; we haven't seen anything." And he added that the repairs appear, "to be working the way we want it to."

The 650-ton steel box was barged to Lake Washington overnight to fix the last two pontoons. Pontoon V is first up for repairs and crews were already preparing it with extra ballast tanks on one end to hold it down when the coffer cell is attached and the extra buoyancy wants to lift it up. WSDOT says the coffer cell will be maneuvered around the end of each pontoon, one end at a time, sealed to the concrete and then the water will be pumped out creating a dry area on the end and underneath the pontoon where workers can complete repairs.

Pontoon U is next to V on the lake and will be the second and last one to be repaired. WSDOT hopes to have all of these repairs finished by June at the latest. Becher says then they'll be able to start joining the pontoons together, "and that's the big milestone we want to get to so we can actually start joining the pontoons and getting back on schedule."

WSDOT admitted last winter that cracks in the end walls of the pontoons were caused by their design error, which put them way off their original intended schedule for completion, and also meant that ultimately the state and the taxpayers pay for the $48 million fix.

And as the Problem Solvers previously reported, the projected overall cost of repairs and overruns for the new bridge have already exceeded the project's contingency fund by over $100 million.